1. Field of the Invention
The present invention relates to a cascaded amplifier with temperature compensation having particular, but not exclusive, application for use as a logarithmic amplifier for dynamic range reduction in radio receiver applications.
2. Description of the Related Art
Piecewise approximations to logarithmic transfer functions are known to be provided by cascaded amplifiers comprising several amplifying stages. A true logarithmic amplifier is the name given to a logarithmic amplifier whose output retains information about the sign of the input signal as opposed to amplifiers which rectify the input signal to provide an output which is logarithmically proportional to the magnitude of the input signal only. Such an amplifier is described in "A True Logarithmic Amplifier for Radar IF Applications" by William L. Barber and Edmund R. Brown in the IEEE Journal of Solid-State Circuits, Vol. SC15, No. 3 , June 1980. The paper describes a logarithmic amplifier which consists of a cascade of dual gain amplifying stages. Each stage comprises a limiting amplifier designed to be driven into limiting at a given input voltage and a unity gain amplifier arranged in parallel with the limiting amplifier. As the input level to the cascaded amplifier rises, the limiting amplifiers in consecutive dual gain stages are driven into limiting, starting with the last one in the chain. The overall gain of the amplifier decreases as the input voltage level rises and a piecewise approximation to a logarithmic function results. The larger the number of amplifier stages, the more accurate is the approximation to the function and/or the greater the input dynamic range with which the amplifier can operate. The small signal gain of each amplifier stage in a typical logarithmic amplifier might be 10 dB.
The amplifier stages described in the above-mentioned article each comprise two long-tailed pair transistor amplifiers driving a common load resistor. One of the long-tailed pairs has degeneration to provide a unity gain amplifier with no limiting and the other has no degeneration (other than that inherent in the transistors) to provide an amplifier having a high gain for small-signal inputs but which limits at larger signal inputs. A cascaded chain of such amplifier stages can be arranged to give a good approximation to a logarithmic transfer function.
The main drawback of this known design of cascaded amplifier is that the amplitude transfer characteristics vary widely with variations in temperature.